EP0343403B1 - Circuit for the self-excitation of a mechanical oscillation system to its characteristic resonant frequency - Google Patents

Description

The invention relates to a circuit arrangement for self-excitation of natural resonance vibrations of the mechanical vibration system of a level sensor with an electromechanical transducer system, which is arranged in the feedback circuit of an electronic amplifier circuit, so that it is excited by the output AC voltage of the amplifier circuit to mechanical vibrations and an AC voltage to the input of the amplifier circuit Provides frequency of mechanical vibrations, wherein the amplifier circuit contains an operational amplifier with a non-linear gain characteristic, which results in a greater gain with small values of the input signal than with larger values of the input signal.

A circuit arrangement of this type is known from EP-A-240 360. The nonlinear gain characteristic solves a problem that occurs in particular in mechanical vibration systems of level sensors. To make a statement the reaching of a predetermined level in a container with the aid of a mechanical oscillation system excited to natural resonance vibrations takes advantage of the fact that the vibrations cease when the sensor is immersed in the product due to the strong damping, while the re-insertion of the vibrations indicates that the level falls below the installation height of the sensor has fallen. If the sensor in the process container is exposed to high temperatures in such an application, the transmission factor of the sensor can change so much that it can no longer oscillate, which leads to an incorrect display of the fill level. In the same way, fillings (e.g. lime, flour) that tend to build up have a strong effect: If the buildup is strong, the sensor can no longer swing, so that it is incorrectly displayed that the sensor is covered, although in reality it is not immersed in the filling and is only covered with neck. If the gain of the amplifier circuit is increased to avoid this problem, the external vibration sensitivity becomes too great. This means that if the sensor is covered, vibrations on the container, which are caused, for example, by vibrators or flowing material, can cause output voltages from the amplifier circuit, which pretend that the sensor is not covered and is carrying out natural resonance vibrations, in which case the level is incorrectly displayed. The nonlinear gain characteristic ensures safe start-up even under unfavorable operating conditions and reduces the risk of incorrect displays of the vibration state without the external vibration sensitivity becoming too great.

In the circuit arrangement known from EP-A-240 360, the feedback circuit of the operational amplifier contains a non-linear resistor with a positive temperature coefficient in series with a series resonant circuit formed from an inductance and a capacitance. The nonlinear amplifier characteristic curve arises from the fact that the nonlinear resistance has a smaller one when the vibrations begin Has resistance value than in the stationary vibration state. In addition, since the mechanical oscillation structure lies in a capacitive bridge, which is coupled to the output of the operational amplifier via a second inductance, this circuit arrangement is complex, and it contains circuit elements which are not used with the usual semiconductor circuit technology.

From the magazine "Wireless World", volume 73, No. 12, December 1967, pages 594-598, diode function generators are known, which are formed by an operational amplifier with a feedback resistor that varies depending on the signal amplitude. In order to achieve the variable feedback resistance, the feedback circuit has a plurality of parallel branches, each of which contains a resistor in series with a diode. The resistance values are graded in such a way that the diodes become conductive when the input signal has different values and then switch on the series resistance in parallel in the feedback branch. This results in a functional characteristic in the form of a broken line. However, this effect only occurs for one of the two current directions, because all diodes are blocked for the other current direction. In order to change an AC voltage according to the functional characteristic, this AC voltage must be set on one side to the reference potential, so that it changes only on one side. This known circuit arrangement is therefore not suitable for the self-excitation of natural resonance vibrations of the mechanical vibration system of a level sensor by means of an alternating voltage that is symmetrical with respect to the reference potential.

The object of the invention is to provide a circuit arrangement of the type specified in the introduction, which contains only circuit elements of conventional semiconductor circuit technology and which enables self-excitation of natural resonance vibrations of the mechanical vibration system of a level sensor by means of an alternating voltage which is symmetrical with respect to the reference potential.

According to a first embodiment of the invention, this object is achieved in that the feedback circuit of the operational amplifier contains two resistors connected in series and that one of the two resistors has two semiconductor diodes connected in parallel in opposite directions.

A second embodiment of the invention is that the inverting input of the operational amplifier is connected to ground through a circuit branch which contains a field effect transistor, and that the current path resistance of the field effect transistor is variable by a control voltage applied to its gate electrode, which is different from the output voltage of the operational amplifier depends.

Fig. 1

das Blockschaltbild einer Schaltungsanordnung zur Erregung eines mechanischen Schwingsystems zu Eigenresonanzschwingungen,

Fig. 2

das Schaltbild einer Ausführungsform des Eingangsverstärkers der Schaltungsanordnung von Fig. 1,

Fig. 3

Diagramme zur Erläuterung der Funktionsweise des Eingangsverstärkers von Fig. 2,

Fig. 4

das Schaltbild einer anderen Ausführungsform des Eingangsverstärkers von Fig. 2 und

Fig. 5

Diagramme zur Erläuterung der Funktionsweise des Eingangsverstärkers von Fig. 4.

Exemplary embodiments of the invention are described below, which are shown in the drawing. The drawing shows:

Fig. 1

the block diagram of a circuit arrangement for exciting a mechanical vibration system to natural resonance vibrations,

Fig. 2

1 shows the circuit diagram of an embodiment of the input amplifier of the circuit arrangement of FIG. 1,

Fig. 3

2 to illustrate the operation of the input amplifier of FIG. 2,

Fig. 4

the circuit diagram of another embodiment of the input amplifier of Fig. 2 and

Fig. 5

4 to illustrate the operation of the input amplifier of FIG. 4.

1 shows, as an example of a mechanical vibration system that is to be excited to vibrate at the natural resonance frequency, a fill level sensor 10 with two vibrating bars 12, 14. The vibrating bars are set into flexural-phase bending vibrations, which are so strongly damped when the bars are immersed in the product that the vibrations cease, whereby it can be determined that the filling material has reached a predetermined filling level, while conversely the re-insertion of the vibrations indicates that the filling level has again fallen below the level to be monitored. The vibrating rods 12, 14 are each fastened at one end to a membrane 16 which is clamped at the edge in a holder 18.

In order to generate the natural resonance vibrations of the mechanical vibration system 10, an electromechanical transducer system 20 is connected to the membrane 16, which has a transmitter transducer 22 and a receiver transducer 24. The transmitter converter 22 is connected to the output of an amplifier circuit 30 and is designed such that it converts an electrical alternating voltage (or an electrical alternating current) supplied by the amplifier circuit 30 into a mechanical oscillation which acts on the membrane 16 and on the oscillating rods 12, 14 is transmitted. The reception converter 24 is connected to the input of the amplifier circuit 30 and is designed such that it converts the mechanical oscillation of the oscillation system 10 into an electrical alternating voltage of the same frequency. This AC input voltage is amplified by the amplifier circuit, and the amplified AC output voltage of the same frequency thus obtained is applied to the transmitter converter 22. It can be seen immediately that the mechanical vibration system is in this way in a self-exciting feedback circuit of the amplifier circuit 30, in which it forms the frequency-determining element, so that it is excited to oscillate at its natural resonance frequency.

The electromechanical transducers 22, 24 can be of any type known per se, for example electromagnetic or electrodynamic transducers with coils, magnetostrictive transducers, piezoelectric transducers or the like. In the described embodiment, it is assumed that it is a piezoelectric transducer which contains, in a known manner, a piezo crystal arranged between two electrodes, which undergoes a change in shape when an electrical voltage is applied to the two electrodes, and vice versa in the case of a mechanically forced one Shape change creates an electrical voltage between the two electrodes. The transmitter converter 22 and the receiver converter 24 can therefore be of the same type.

The amplifier circuit 30 contains an input amplifier 32, the input terminals of which are connected to the two electrodes of the receiving transducer 24, a bandpass filter 34 connected to the output of the input amplifier 32, and a power amplifier 36, the output electrodes of which are connected to the two electrodes of the transmitter transducer 22. The bandpass filter 34 is tuned to the natural resonance frequency of the electromechanical oscillation system 10 to be excited, so that the electrical AC voltage is selectively amplified with this frequency. This can be the frequency of the fundamental oscillation or the frequency of a harmonic of the natural resonance of the mechanical oscillation system 10.

The peculiarity of the amplifier circuit 30 is that its gain characteristic, depending on the size of the input signal, is so non-linear that the amplification is greater for small amplitudes of the input signal than for large amplitudes. In the illustrated embodiment, this non-linear gain characteristic of the amplifier circuit 30 is achieved in that the input amplifier 32 is designed with a non-linear gain.

Fig. 2 shows an embodiment of the input amplifier 32, which gives the desired non-linear gain characteristic with particularly simple means. The input amplifier 32 is designed as a differential amplifier with an operational amplifier 40. The two inputs of the operational amplifier 40 are connected via identical resistors 41, 42 of the resistance value R 1 to the two electrodes of the receiving transducer 24, so that the voltage between these electrodes forms the input voltage U _{e of} the differential amplifier. In the leading from the output to the inverting input feedback branch of the operational amplifier 40 are two resistors 43, 44 with the resistance values R₂ and R₃ in series, and two further resistors 45, 46 with the same resistance values R₂ and R₃ are in series between the non-inverting input of the operational amplifier 40 and ground connected. Two semiconductor diodes 47, 48 are connected in parallel in opposite directions to resistor 44, and in a corresponding manner two further semiconductor diodes 49, 50 are connected in parallel in opposite directions to resistor 46.

Diejenigen Komponenten der Ausgangsspannung U_a, deren Frequenzen im Durchlaßbereich des Bandfilters 34 liegen, gelangen zum Endverstärker 36, von dem sie mit linearer Verstärkung weiter verstärkt werden. Die so verstärkten Signalkomponenten werden vom Sendewandler 22 in mechanische Schwingungen umgewandelt, die das mechanische Schwingsystem 10 zu einer Eigenresonanzschwingung anregen. Diese Eigenresonanzschwingung wird vom Empfangswandler 24 in eine elektrische Wechselspannung umgesetzt, die dem Eingang des Differenz-Eingangsverstärkers 32 zugeführt und von diesem in der zuvor beschriebenen Weise verstärkt wird. Auf diese Weise schaukeln sich die Schwingungen des mechanischen Schwingsystems 10 auf.The differential amplifier shown in FIG. 2 has the following mode of operation:
If the mechanical oscillation system 10 is at rest when the device is switched on, the receiving transducer 24 initially only emits very small voltages which are caused by slight external vibrations, thermal noise and similar interference effects. These small voltages are amplified by the differential input amplifier 32. As long as the resulting output voltage U _{a of} the differential input amplifier is so small that the voltage drops across the resistors 44 and 46 are smaller than the forward voltage of the semiconductor diodes 47, 48, 49, 50 (which is about 0.6 V for silicon diodes ), the semiconductor diodes block in both directions and the resistors 44 and 46 are fully effective. For such small input signals, the gain factor V of the differential input amplifier is 32

Those components of the output voltage U _a whose frequencies are in the pass band of the bandpass filter 34 reach the final amplifier 36, from which they are further amplified with linear amplification. The signal components amplified in this way are converted by the transducer 22 into mechanical vibrations, which excite the mechanical vibration system 10 to produce a natural resonance vibration. This natural resonance oscillation is converted by the receiving converter 24 into an electrical AC voltage, which is fed to the input of the differential input amplifier 32 and is amplified by it in the manner described above. In this way, the vibrations of the mechanical vibration system 10 rock up.

Das Diagramm A von Fig. 3 zeigt diese Abhängigkeit des Verstärkungsfaktors V von der Spannung, und das Diagramm B von Fig. 3 zeigt den dadurch erzielten Zusammenhang zwischen der Eingangsspannung U_e und der Ausgangsspannung U_a des Eingangs-Differenzverstärkers 32. Bei Werten der Eingangsspannung U_e, die kleiner als ein Wert U_e1 sind, ist die Ausgangsspannung U_a durch den konstanten Verstärkungsfaktor V₁ bestimmt, so daß sie mit verhältnismäßig großer Steilheit der Eingangsspannung U_e proportional ist. In diesem Bereich besitzt die Verstärkerschaltung 30 eine große Eingangsempfindlichkeit, so daß selbst bei schwachen Störeffekten sowie bei temperaturbedingten Änderungen des Übertragungsfaktors und bei Ansatzbildungen an den Schwingstäben 12, 14 ein sicheres Anschwingen gewährleistet ist.If in this transient process the voltage U _a at the output of the differential input amplifier 32 becomes so great that the voltage drops across the resistors 44 and 46 become greater than the forward voltage of the semiconductor diodes 47, 48 and 49, 50, respectively, the semiconductor diodes become transparent that they short out resistors 44 and 46. The gain factor V of the differential input amplifier 32 is then

The diagram A in FIG. 3 shows this dependence of the gain factor V on the voltage, and the diagram B in FIG. 3 shows the relationship between the input voltage U _e and the output voltage U _{a of} the input differential amplifier 32 that is achieved. For values of the input voltage U _e , which are smaller than a value U _e1 , the output voltage U _{a is} determined by the constant gain factor V 1, so that it is proportional to the input voltage U _e with a relatively high steepness is. In this area, the amplifier circuit 30 has a high input sensitivity, so that a reliable start-up is ensured even with weak interference effects and with temperature-related changes in the transmission factor and with build-ups on the oscillating rods 12, 14.

At the value U _{e1 of} the input voltage U _e , the output voltage U _a reaches a value U _a1 due to the amplification with the amplification factor V₁, which is equal to the forward voltage of the semiconductor diodes 47, 48, 49, 50. At values of the input voltage U _e that are greater than the value U _e1 , the gain factor V therefore has the smaller value V₂, so that the output voltage U _a rises less steeply as _a function of the input voltage U _e . In this area, in which there are no start-up problems, the input sensitivity of the amplifier circuit is therefore reduced, so that voltages which are generated by interference vibrations cannot reach values which simulate a resonant vibration of the mechanical vibration system 10.

When the input voltage U _e finally reaches a value U _e2 at which the output voltage U _{a has} the maximum value U _B caused by the power supply voltage, the input amplifier 32 goes into saturation, so that a further increase in the input voltage U _e does not increase the output voltage U _{a results in} more.

The effects described are achieved with very little additional circuitry. Compared to a differential input amplifier with linear amplification, the additional effort is limited to the two resistors 44, 46 and the four semiconductor diodes 47, 48, 49, 50.

Somit wird am Ausgang des Operationsverstärkers 40 die Spannung

${\text{U}}_{\text{a}}{\text{′ = U}}_{\text{e}}{\text{·V}}_{\text{I}}\text{ (4)}$

abgegeben.Fig. 4 shows another embodiment of the input amplifier 32, which also gives the desired non-linear gain characteristic. In this embodiment, the Input amplifier 32 from two amplifier stages. The first amplifier stage corresponds to the input amplifier of FIG. 2 with the only difference that the resistors 44 and 46 with the semiconductor diodes 47, 48 and 49, 50 connected in parallel in opposite directions are omitted. The remaining components of this amplifier stage, which correspond to those of the input amplifier of FIG. 2, are designated by the same reference numerals as in FIG. 2. As in Fig. 2, the two electrodes of the receiving transducer 24 are connected via the same resistors 41, 42 of the resistance value R 1 to the two inputs of the operational amplifier 40, so that the voltage between these electrodes forms the input voltage U _{e of} the differential amplifier. Now that only the unchangeable resistors 43 and 45 of the resistance value R₂ are in the feedback circuit of the operational amplifier 40 and in the circuit branch leading from the non-inverting input to ground, this amplifier stage has the constant gain factor

Thus, the voltage at the output of operational amplifier 40

${\text{U}}_{\text{a}}{\text{′ = U}}_{\text{e}}{\text{· V}}_{\text{I.}}\text{(4)}$

submitted.

der in Abhängigkeit von dem Widerstand R_FET und somit in Abhängigkeit von der Ausgangsspannung U_a veränderlich ist.The second amplifier stage contains an operational amplifier 60, the non-inverting input of which is connected to the output of the first amplifier stage, so that the output voltage U _a 'of the first amplifier stage forms the input voltage of the second amplifier stage, the output voltage U _{a of which also} represents the output voltage of the input amplifier 32. In the feedback circuit of the operational amplifier 60 leading to the inverting input there is a resistor 61 with the resistance value R₄. Furthermore lies between the inverting Input of the operational amplifier 60 and ground, a circuit branch which contains a resistor 62 with the resistance value R₅ in series with the current path of a field effect transistor 63. The resistance R _{FET of} the field effect transistor 63 depends on the control voltage applied to its gate electrode. This control voltage is obtained from the output voltage U _a by rectification by means of a rectifier circuit which contains two semiconductor diodes 64, 65 and a smoothing circuit with a capacitor 66 in parallel with a resistor 67. Thus, the current path resistance R _{FET of} the field effect transistor 63 is dependent on the amplitude of the output voltage U _a . This results in the gain factor for the second amplifier stage

which is variable as _a function of the resistor R _FET and thus as a function of the output voltage U _a .

Der aus den beiden Verstärkerstufen bestehende Eingangsverstärker 32 hat den Gesamtverstärkungsfaktor V_G

${\text{V}}_{\text{G}}{\text{= V}}_{\text{I}}{\text{·V}}_{\text{II}}\text{, (7)}$

so daß zwischen der Eingangsspannung U_e und der Ausgangsspannung U_a des Eingangsverstärkers 32 die folgende Beziehung besteht:

${\text{U}}_{\text{a}}{\text{= U}}_{\text{e}}{\text{·V}}_{\text{G}}\text{ (8)}$

Die Zusammenhänge zwischen den Verstärkungsfaktoren V_I, V_II, V_G und den Spannungen U_e, U_a′, U_a sind in den Diagrammen von Fig. 5 dargestellt.The gain factor V _II determines the relationship between the input voltage U _a 'and the output voltage U _{a of} the second amplifier stage

${\text{U}}_{\text{a}}{\text{= U}}_{\text{a}}{\text{'· V}}_{\text{II}}\text{(6)}$

The input amplifier 32 consisting of the two amplifier stages has the total amplification factor V _G

${\text{V}}_{\text{G}}{\text{= V}}_{\text{I.}}{\text{· V}}_{\text{II}}\text{, (7)}$

so that the following relationship exists between the input voltage U _e and the output voltage U _{a of} the input amplifier 32:

${\text{U}}_{\text{a}}{\text{= U}}_{\text{e}}{\text{· V}}_{\text{G}}\text{(8th)}$

The relationships between the amplification factors V _I , V _II , V _G and the voltages U _e , U _a ', U _a are shown in the diagrams in FIG. 5.

In Fig. 5, the diagram A shows the voltage-dependent course of the gain factor V _I and the diagram B shows the relationship between the input voltage U _e and the output voltage U _a 'of the first amplifier stage. Up to a value U _{e2 of} the input voltage at which the output voltage U _a 'reaches the saturation value U _B , the gain factor V 1 is constant, so that the voltage U _a ' is proportional to the input voltage U _e .

The diagrams C and D show the conditions for the second amplifier stage in a corresponding manner. Up to a value U _a1 'of the voltage U _a' has the amplification factor V _II a relatively large constant value V _II1, so that the output voltage U _a of the voltage U _a 'with a relatively large slope is proportional. Between the values U _a1 and U _a2 'of the input voltage U _a ' and the corresponding values U _a1 and U _{a2 of} the output voltage U _a is the range of change of the resistance R _FET ; consequently, the amplification factor V _II in this area drops from the value V _II1 to a lower value V _II2 , which results in the nonlinear relationship shown in the diagram D between the voltages U _a 'and U _{a in} this area. Between the voltage value U _a2 'and a voltage value U _a3 ', at which the output voltage U _{a reaches} the saturation value U _B , the resistance R _FET no longer changes, so that in this area the gain factor V _{II maintains} the constant lower value V _II2 and the voltage U _{a is} again proportional to the voltage U _a ', but with a significantly lower slope.

Finally, diagram E shows the total amplification factor V _{G of} the input amplifier 32, which results from the product of the two amplification factors V _I and V _II , and diagram F shows the corresponding relationship between the Input voltage U _e and the output voltage U _a . It can be seen immediately that diagram F of FIG. 5 is very similar to diagram B of FIG. 3. In particular, also in the embodiment of FIG. 4, the input amplifier has a large amplification factor and therefore a high input sensitivity for small values of the input voltage U _e , while the amplification factor is smaller for higher values of the input voltage and consequently the input sensitivity is reduced. The embodiment of FIG. 4 therefore gives the same advantageous effects as were previously explained for the embodiment of FIG. 2.

Claims (4)

1. Circuit arrangement for self-excitation of natural resonant oscillations of the mechanical oscillation system (10) of a filling level sensor comprising an electromechanical transducer system (20-24) which is arranged in the feedback circuit of an electronic amplifier circuit (30) so that said system is stimulated by the output AC voltage (U_a) of the amplifier circuit to mechanical oscillations and furnishes an AC voltage (U_e) with the frequency of the mechanical oscillations to the input of the amplifier circuit, the amplifier circuit (30) comprising an operational amplifier (40) having a non-linear gain characteristic which at small values of the input signal (U_e) gives a greater gain that at larger values of the input signal, characterized in that the feedback circuit (43, 44, 47, 48) of the operational amplifier (40) contains two resistors (43, 44) connected in series and that with one (44) of the two resistors two semiconductor diodes (47, 48) are connected in parallel in opposite senses.
2. Circuit arangement according to claim 1, characterized in that the operational amplifier (40) is formed as differential amplifier with two additional resistors (45, 46) connected in series between the non-inverting input and ground and that two semiconductor diodes (49, 50) are connected in parallel in opposite senses with one of the additional resistors (46).
3. Circuit arrangement for self-excitation of natural resonant oscillations of the mechanical oscillation system (10) of a filling level sensor comprising an electromechanical transducer system (20-24) which is arranged in the feedback circuit of an electronic amplifier circuit (30) so that said system is stimulated by the output AC voltage (U_a) of the amplifier circuit to mechanical oscillations and furnishes an AC voltage (U_e) with the frequency of the mechanical oscillations to the input of the amplifier circuit, the amplifier circuit (30) comprising an operational amplifier (60) having a non-linear gain characteristic which at small values of the input signal gives a greater gain than at larger values of the input signal, characterized in that the inverting input of the operational amplifier (60) is connected to ground via a circuit branch containing a field-effect transistor (63) and that the current flowpath resistance of the field-effect transistor (63) is variable by a control voltage which is applied to the gate electrode thereof and which depends on the output voltage (U_a) of the operational amplifier (60).
4. Circuit arrangement according to claim 3, characterized in that the control voltage is formed from the output voltage by rectification.

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